Conventional mechanical vernier caliper is gradually replaced with the electronic digital display caliper. The electronic digital display caliper most widely used at present uses capacitive displacement sensors, and the output of the sensor is associated with the relative positions of the capacitor plates on the main ruler and the secondary ruler. With the processing of the electronic circuit, the data of the relative positions is displayed in the display screen. The electronic digital display caliper in this principle is described in SE7714010 and European Patent EP0053091. Although the electronic digital display caliper in this principle has high precision and low power consumption, there is a specific requirement for application environment of the caliper. The caliper will work abnormally in wet environment or in the environment with lubrication and dust and the like. However, it is difficult to avoid such environment in practice, which will limit the application range of such kind of electronic digital display caliper.
In order to allow the electronic digital display caliper to work normally in wet environment and in the environment with lubrication and dust and the like, one solution is to measure displacement by using a magnetic displacement sensor. The magnetic displacement measurement method has been described in U.S. patents of U.S. Pat. Nos. 4,226,024, 4,612,502 and 5,174,041. However, the structure disclosed in the above patents is complicated and has high power consumption and low precision, which is not usable as a portable measuring tool. Chinese Patent Application No. CN1254412A discloses a magnetic portable electronic digital display caliper, in which the magnetic displacement sensor uses such a structure as shown in FIG. 3, wherein a main ruler 103 is provided on the ruler body 101; a plastic film 134 is provided on the surface of the main ruler; the main ruler 103 uses substrate made of magnetic material and is magnetized at constant intervals in the movement direction so as to form grid-type magnetic poles, thereby the magnetic field intensity on the surface of the main ruler 103 varies with the period of grid pitch λ; a substrate 141 is fixed on a vernier corresponding to the main ruler 103 and a magnetoresistance 142 is provided on the substrate 141; said magnetoresistance 142 detects the magnetic field intensity on the surface of the main ruler 103 so as to realize the measurement of position. Because it is not easy to make the magnetization intensity of the main ruler 142 is in the form of single layer film, the resistance change rate is small, which would impose high requirement on the measurement circuit. In addition, the bridge used to detect the resistance change is a DC bridge; and the displacement amount is calculated according to the amplitude output from the bridge. Therefore, this method will bring displacement float due to external electronic interference and environment variation, eventually will influence the measurement precision.